GMT_IMAGE: Remove 'nan_value' from the doctest since it depends on GMT versions

[seisman]

Description of proposed changes

In the "GMT Legacy Tests" workflow, we have one failure:

=================================== FAILURES ===================================
__________________ [doctest] pygmt.datatypes.image._GMT_IMAGE __________________
012 
013     GMT image data structure.
014 
015     Examples
016     --------
017     >>> import numpy as np
018     >>> from pygmt.clib import Session
019     >>> with Session() as lib:
Differences (unified diff with -expected +actual):
    @@ -9,4 +9,4 @@
     0 3 364 184
     [2, 2, 2, 2]
    -b'BRPa' 0.0 0.5
    +b'BRPa' nan 0.5
     1 0

/home/runner/work/pygmt/pygmt/pygmt/datatypes/image.py:19: DocTestFailure
----------------------------- Captured stderr call -----------------------------
Warning: sion [WARNING]: Remote dataset given to a data processing module but no registration was specified - default to gridline registration (if available)

The image.nan_value is 0.0 in GMT 6.5 and is nan in old versions. It's likely related to upstream changes in GenericMappingTools/gmt#7827, but I guess we don't care.

This PR remove header.nan_value from the doctest so it passes for GMT 6.3-6.5. I also change @earth_day_01d to @earth_day_01d_p to silent the warning.

Triggered CI runs at: https://github.com/GenericMappingTools/pygmt/actions/runs/10287846021